1. Field of the Invention
The present invention relates, in general, to wet flue gas desulfurization (WFGD) systems and, in particular, to a new and useful method of reducing the pressure drop in a downflow/upflow WFGD system and improving its collection efficiency by converting it to an upflow single-loop WFGD system.
2. Description of the Related Art
The desulfurization of flue gas, particularly flue gas from power plants, has been the subject of considerable study. Air quality laws, both at the federal and state level, have set increasingly stringent emission standards especially for such known pollutants as sulfur oxides. Because coal and oil-fired electrical power generating plants can discharge large quantities of sulfur oxides as combustion by-products, much effort has focused on the desulfurization of flue gas to reduce power plant sulfur dioxide emissions to permissible levels.
Thus, sulfur oxides, principally present as sulfur dioxide, are found in the flue gases discharged by coal and oil-fired and other fossil fuel-fired electrical power generating plants, refuse-to-energy plants, and the waste gases from other industrial processes. In addition, sulfur-containing gases, notably sulfur dioxide, may be formed in the partial combustion or gasification of sulfur-containing fuels, such as coal or petroleum residuals. The control of air pollution resulting from the discharge of sulfur dioxide into the atmosphere has thus become increasingly urgent.
The most common flue gas desulfurization process used with coal and oil-fired electrical generating power plants is known as xe2x80x9cwet scrubbingxe2x80x9d. In this process the sulfur dioxide-containing flue gas is scrubbed with an aqueous alkaline solution or slurry reagent comprised of lime, limestone, soda ash, or other chemicals including sodium, magnesium and calcium compounds and may include any number of additives to enhance removal, control chemistry, and reduce chemical scale.
The technology for wet scrubbing provides gas-liquid contact in a number of differently configured systems. One of the more prominent of these systems is comprised of a downflow quencher and an upflow absorber. The hot flue gas to be treated enters the quencher which is equipped with a venturi scrubber or spray headers connected to a slurry or water source to produce droplets that promote rapid cooling of the hot flue gas as it flows downwardly through the quencher. After leaving the quencher, the cooled flue gas discharges into a lateral passageway and flows therethrough and then upwardly through the absorber where it is scrubbed with an alkaline slurry reagent where the gas flow is countercurrent to and in intimate contact with the slurry reagent. The slurry reagent is introduced into the absorber through spray header nozzles and flows over packing or trays. Mist eliminators are included near the absorber outlet to remove additional moisture from the flue gas.
While the downflow/upflow WFGD system generally provides the sulfur dioxide removal effect, it experiences a pressure loss higher than that of a contemporary single-loop WFGD system of the same capacity. It, then, follows that the downflow/upflow WFGD system requires more fan power and more pump power than the single-loop WFGD system. This, in turn, increases the operating and maintenance costs of a downflow/upflow WFGD system when compared to a single-loop WFGD system of the same capacity.
In other words, the present invention makes it possible to decrease the flow resistance of the flue gas and thereby reduce the operating and maintenance costs.
As noted, the trend in pollution control has been towards increased stringency, such that many facilities face the need to upgrade or retrofit their existing pollution control equipment to achieve better performance. In addition owners/operators are often interested in upgrading or retrofitting existing pollution control equipment to realize the benefit of lower operational and maintenance costs from improved efficiency. In many situations, the retrofitting or upgrading of an air pollution control system is difficult due to space and/or power consumption considerations. A benefit of the present invention is that it addresses both of these conditions by conforming the retrofit to the existing space and by lowering fan power and pump power requirements through a decrease in pressure loss across the pollution control system, and improved effectiveness in the removal of sulfur dioxide from the flue gases. The present invention can provide pressure drop reductions across the system of about 5 inches water gage.
The present invention provides a method of reducing the pressure drop in a downflow/upflow WFGD system by converting it to an upflow single-loop WFGD system. The downflow/upflow system includes a downflow quencher and an upflow absorber and a lateral flow passageway therebetween. The downflow quencher is comprised of a venturi scrubbing device mounted in the duct work used to convey the incoming flue gas through the quencher for discharge into the lateral passageway for flow therethrough to the absorber. As a practical matter, venturi scrubbing devices, even those claimed to utilize very fine droplets, actually utilize droplets which are much larger than the optimal size. The primary methods heretofore utilized in improving the collection efficiency of a venturi scrubber have been to decrease the size of the throat or to increase the overall rate at which gas flows through the system. Both of these methods increase the differential velocities between the contaminant particles and the liquid droplets as they pass through the throat of the venturi scrubber This causes more interactions between particles and droplets to occur, thereby improving contaminant removal. However, increasing the collection efficiency in this manner comes at a cost of significantly higher energy input into the system, thereby resulting in higher operating costs. The extra energy is expended due either to the increased overall resistance attributable to the reduced throat diameter or to the increased overall gas flow rate through the venturi scrubber. In either case, the pressure drop across the venturi is increased and greater fan and pumping capacity is required.
The method according to the present invention replaces the duct work, the quencher and, except for an alternate embodiment hereinafter described, the lateral passageway with a bypass that conveys the incoming flue gas directly to the absorber. The quenching zone is transferred to the absorber and replaced by a spray level. The spray level includes a plurality of spray nozzles mounted on headers arranged parallel to one another. The nozzles spray an aqueous slurry of sulfur dioxide-reducing reagent within the spray zone to contact the flue gas while descending through the absorber counter-currently to the flow of flue gas, the slurry reagent is collected in the absorber sump or reaction tank and a portion of it is recycled for contact with the flue gas flowing through the absorber. The piping used to supply the slurry reagent to the quencher in the replaced duct work may be rerouted to the spray nozzle headers located in the absorber. The replacement of the bypassed quencher with a level of spray nozzles improves overall sulfur dioxide removal from the flue gases flowing through the system. An awning is mounted over the absorber inlet to prevent the slurry reagent from entering the inlet, and to initially deflect the incoming flue gas in a downward direction thereby achieving a more uniform distribution of the flue gas in its upward flow through the absorber. The bypass is configured to have a lesser number of turns than the duct work thereby reducing pressure losses. The front wall of the absorber is extended below the absorber inlet and becomes the front wall of the sump so as to accommodate the replacement of the lateral passageway with the bypass and the connecting of the bypass with the absorber. An overflow conduit is added to the front wall of the sump to maintain a desired or preset level of slurry reagent and contaminant particles in the sump, with any excess slurry reagent and contaminant particles being discharged through downcomers to a holding tank. The bypass, the awning, and the front wall of the sump are fabricated from alloys that are corrosion-resistant to both oxidizing and reducing media, and are resistant to localized corrosion attack.
An optional standby quencher may also be provided in the bypass for emergency use.
Flow guide elements may be mounted in the bypass such as turning vanes around corners so as to promote laminar flow of the flue gases, particularly around sharp corners in the duct work, and thus further reduce pressure losses.
The lateral passageway need not be replaced, provided that it is restructured in that its flue gas inlet opening located on the roof is closed off and replaced by a flue gas inlet opening on the front wall. The bypass is then connected to the portion of the passageway front wall bordering the relocated flue gas inlet opening. A set of headers and nozzles may have to be added on the gas side of the passageway roof as part of the restructuring so as to provide a spray of alkaline solution to primarily prevent the overheating of the roof.
It should be noted that removal of the duct work and the venturi scrubber type quencher will not only reduce fan power and pump power requirements due to reduced pressure drop across the system, but also result in the elimination of the costly maintenance associated with the venturi scrubber throat.